Write a method/function that reads in a text file and returns a list of the tokens in that file. For the purposes of this project, a token is a sequence of alphanumeric characters, independent of capitalization (so Apple, apple, aPpLe are the same token). You are allowed to use regular expressions if you wish to (and you can use some regexp engine, no need to write it from scratch), but you are not allowed to import a tokenizer (e. g. from NLTK), since you are being asked to write a tokenizer.